RTOS API for bare metal ...

Provide partial RTOS API for bare metal builds - things that
can be done in a single threaded environment.

Allows more code to work in both RTOS and bare metal builds without
change, and in particular gives easy access to the ability to
efficiently wait for something occurring in interrupt.

Available in bare-metal:
* ThisThread
* osThreadFlagsSet to set flags on main thread (can be set from IRQ)
* EventFlags (can be set from IRQ)
* Semaphores (can be released from IRQ)
* Mutex (dummy implementation)

Not useful:
* ConditionVariable (could only be signalled from 2nd thread)
* RtosTimer (calls in a second thread context)
* Thread

Unimplemented:
* Mail, Queue, MemoryPool

Possible future work:
* ConditionVariableCS to act as IRQ signalled ConditionVariable

Introduce Semaphore::acquire methods ...

Deprecate wait() in favour of acquire(), try_acquire(),
try_acquire_for() and try_acquire_until().

Brings Semaphore more into line with CMSIS-RTOS 2 (which uses "acquire"),
itself (as it has "release"), and other classes having "try", "try for"
and "try until".

Also steps away from vague "wait" term - the primary operation here is
to acquire the semaphore, and this will of course sleep.

Change EventFlag timeout paramter ...

Matches rest of RTOS class timeout parameters by using the unit name. Remove ambigious statement in reference to 0 ms being no-timeout as a timeout of 0 causes the function to not block and return immediately (osWaitForever is used as no timeout as it will wait forever)

rtos: fix astyle

Add Semaphore::wait_until ...

Given the 64-bit timebase, add wait_until to Semaphore.

Naming is based on Thread::wait_until.

pthreads uses "timedwait", but that's not a good fit against our
existing wait() - pthreads only has an absolute-time wait, not relative.

Copy edit Semaphore.h ...

Copy edit file for active voice.

Add notes about ISR safety to RTOS doxygen

Merge pull request #5372 from SenRamakri/sen_PlatformDoxygenUpdates ...

Doxygen comments fixes

Shrink RTOS classes ...

Various RTOS classes were storing their CMSIS-RTOS creation attribute
structure as a member, when it's not required after construction. Reduce
memory by eliminating this member.

Doxygen comment updates and fixes

RTOS: Semaphore: Improve API docs

Test: RTOS: Semaphore: Rework tests

rtos: Tag non value type as NonCopyable. ...

The types marked are: Mail, MemoryPool, Mutex, Queue, RtosTimer and Semaphore.

Resolve Doxygen errors from RTOS. Fixed minor Doxygen bug in FileSystemHandle.h as well

Update codebase for CMSIS5/RTX5 ...

Update all of mbed-os to use RTX5.

Add tags to our code

restructure - Restructured rtos directory